docs: def Tandon_Varma_Gupta(x, rhol, rhog, mul, mug, m, D):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_ also given in [2]_, [3]_, and [4]_.
  
      For 50 < Rel < 1125:
      
      .. math::
          \alpha = 1- 1.928Re_l^{-0.315}[F(X_{tt})]^{-1} + 0.9293Re_l^{-0.63}
          [F(X_{tt})]^{-2}
      
      For Rel > 1125:
      
      .. math::
          \alpha = 1- 0.38 Re_l^{-0.088}[F(X_{tt})]^{-1} + 0.0361 Re_l^{-0.176}
          [F(X_{tt})]^{-2}
      
      .. math::
          F(X_{tt}) = 0.15[X_{tt}^{-1} + 2.85X_{tt}^{-0.476}]
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      mul : float
          Viscosity of liquid [Pa*s]
      mug : float
          Viscosity of gas [Pa*s]
      m : float
          Mass flow rate of both phases, [kg/s]
      D : float
          Diameter of the channel, [m]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      [1]_ does not specify how it defines the liquid Reynolds number. 
      [2]_ disagrees with [3]_ and [4]_; the later variant was selected, with:
      
      .. math::
          Re_l = \frac{G_{tp}D}{\mu_l}
      
      The lower limit on Reynolds number is not enforced.
          
      Examples
      --------
      >>> Tandon_Varma_Gupta(.4, 800, 2.5, 1E-3, 1E-5, m=1, D=0.3)
      0.9228265670341428
  
      References
      ----------
      .. [1] Tandon, T. N., H. K. Varma, and C. P. Gupta. "A Void Fraction Model 
         for Annular Two-Phase Flow." International Journal of Heat and Mass 
         Transfer 28, no. 1 (January 1, 1985): 191-198. 
         doi:10.1016/0017-9310(85)90021-3.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      .. [4] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
         Fraction Correlations for Different Flow Patterns in Horizontal and 
         Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
         no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
      '''
      G = m/(pi/4*D**2)
      Rel = G*D/mul
      Xtt = Lockhart_Martinelli_Xtt(x, rhol, rhog, mul, mug)
      Fxtt = 0.15*(Xtt**-1 + 2.85*Xtt**-0.476)
      if Rel < 1125:
          alpha = 1 - 1.928*Rel**-0.315/Fxtt + 0.9293*Rel**-0.63/Fxtt**2
      else:
          alpha = 1 - 0.38*Rel**-0.088/Fxtt + 0.0361*Rel**-0.176/Fxtt**2
      return alpha